If you want an F620, there's a very reliable way. Find a race marker near a large parking lot (I tested this using the Pier 2 Pier race in the parking lot next to the pier, lvl 30+ for it to show up, but it worked every time).

Walk into race marker, host race, select Coupes, start the race on your own, scroll through the cars until you reach the F620, then just quit. No worries about becoming a bad sport if you're hosting and alone.

When you're sent back into free roam, there's your F620 in the parking lot. I imagine you can do this to spawn just about any car you'd like.

Edit: Yeah, it works for pretty much anything; this was after quitting a race with the Bullet selected: